U.S. Pat. Nos. 3,620,933, 3,666,549 and 3,698,940 describe novel ways whereby the surface of nonconductive resin substrates are treated to produce a high energy condition such that the substrate can then be readily metal plated by electroless plating techniques, or otherwise bonded to for other purposes. The methods described in these patents are especially useful in the manufacture of printed circuits.
One method, which is called the "totally additive" method, involves catalyzing the entire substrate, drying this catalyzed substrate, and applying an organic polymer resist coating directly on the substrate surface to mask out or define the non-circuit areas where deposition of metal is unwanted. The resulting catalyzed and masked panel is then treated with a dilute acid or base solution to reactivate or accelerate the unmasked circuit areas, and the panel is then electrolessly plated to build up a desired thickness of metal in those unmasked areas.
Although commercially attractive, it was found that problems of low surface resistivity in the finished circuit board arise when using this totally additive process under practical production conditions, following the usual procedure outlined above. Indeed, if the board is catalyzed in a so-called one-step procedure using a mixed tin-palladium chloride solution, then dried and coated with an organic polymer resist in defining the non-circuit areas, and subsequently accelerated and electrolessly plated, poor insulation resistance between electrically isolated conductive metal portions on the finished circuit board can occur.